The present invention relates to a power conditioner for a DC power supply for a fieldbus network and, more particularly, to an adaptive current source to isolate a DC power supply from a communication bus simultaneously carrying digital communication signals and DC power to attached devices.
In a typical industrial plant application, sensors measure position, motion, pressure, temperature, flow, and other parameters related to the operation of process machinery and activities. Actuators, such as valves and motor controllers, control the operation of the machinery and process activities. The sensors and actuators are remotely located from the human and computerized controllers which gather information from the sensors and direct operation of the actuators. A communication network links the controllers with the sensors and actuators located in the field.
Heretofore, communication between controllers and remote sensors and actuators in industrial applications has been by means of analog signaling. The prevailing standard for analog networking of field devices and the control room in industrial applications has been the Instrument Society of America standard, ISA S50.1. This ISA standard provides for a two wire connection between the controller and each field device. One wire of the system carries the analog signal between the remote device and the controller. The analog signal may be converted to a digital signal useful to a computerized controller. The second wire of the circuit supplies DC power for operation of the remote sensor or actuator.
Communication utilizing digital signaling reduces the susceptibility of the communication system to noise and provides a capability for conveying a wide range of information over the communication network. Digital communication also permits several different devices to communicate over a single pair of wires. Remote devices used in connection with a digital communication system typically incorporate local xe2x80x9cintelligence.xe2x80x9d This permits sensors and actuators to perform diagnostic, control, and maintenance functions locally. Further, the local intelligence permits the devices to communicate directly with each other and perform some functions without the necessity of involving a central control facility, thus promoting the development of distributed control systems.
Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. One such fieldbus is defined by the Instrument Society of America standard, ISA SP50.02. This system utilizes a two wire bus to provide simultaneous digital communication and DC power to remotely located devices.
While fieldbus installations are as varied as the industrial applications with which they are used, an exemplary fieldbus installation is illustrated in FIG. 1. A twisted pair cable, referred to as the home run 2, connects a digital control system 4 and a DC power supply 6 with a number of devices 8 (actuators, sensors and local controllers) in the field. The digital control system 4 and the DC power supply 6 may be located in a control room 10. On the other hand, the power supply 6 could be located in the field or at a marshaling panel. The development of the digital fieldbus may also mean that controllers are located in the field. Several devices 8 can be connected to the home run 2 by spur cables 14 at a terminal referred to as a chicken foot 12 which incorporates signal termination for the home run. In addition, devices can be connected along the home run cable 2 with spur cables 14 that are connected to the home run by spur connectors 13.
Termination is required at each end of the home run 2. To send a digital signal over the network, a connected device varies the amount of current it draws from the bus. The terminators 16 comprise a resistor 18 of approximately the characteristic impedance of the wires of the bus and a capacitor 20 connected in series across the pair of wires of the home run cable 2. When the transmitting device varies the current drawn from the bus, the voltage drop across the terminating resistor 18 varies producing the varying voltage of the digital signal. The capacitor 20 of the terminator 16 prevents dissipation of the DC power through the terminator resistor 18 while permitting transmission of the high frequency digital signal on the bus. In addition, the terminators 16 serve to prevent signals from reflecting from the ends of the home run wires 2.
A power conditioner 22 is necessary to isolate the DC power supply 6 from the bus. The constant voltage power supply 6 will attempt to maintain a constant output voltage. This would prevent the propagation of the varying voltage of the digital signal if the power supply 6 was connected directly to the bus. Typically, the power conditioner 22 comprises an inductor and a resistor in series with the output of the power supply 6. The inductor allows a DC voltage to be conducted to the bus, but blocks the high frequency digital communication signals from the output of the power supply 6. The combination of an inductor in the power conditioner 22 and the capacitors 20 in the terminators 16 may make the network xe2x80x9cringxe2x80x9d or oscillate disrupting signals on the network. A resistor in series with the inductor of the power conditioner 22 is used to critically damp the circuit to prevent oscillation. While a power conditioner 22 based on an inductor and resistor is effective in isolating the DC power supply, if a reasonable amount of current is to be supplied to the network the inductor must be physically large to avoid saturation of the magnetic material from which it is constructed and, as a consequence, will be costly. In addition, the DC current for the network flows through the series resistor of the power conditioner 22 dissipating power which would otherwise be available to the devices connected to the network.
Further, it may be desirable to provide multiple power supplies, particularly for large networks with long wiring runs. The resistance of the long wiring runs may dissipate sufficient power to make powering the network from one power source impractical. The tendency of the inductor isolated circuit to ring and the difficulty of designing a circuit with muWiple inductor-based isolators makes multiple power sources extremely difficult to incorporate in the fieldbus network.
What is desired, therefore, is a compact and efficient apparatus, adaptable to the power needs of a wide variety of installations, for isolating the output a DC power supply from voltage fluctuations on a bus simultaneously carrying digital communication signals and DC power.
The present invention overcomes the aforementioned drawbacks of the prior art by providing an apparatus for isolating the output of a constant voltage power source from a voltage variation in an electrical circuit connected to the output comprising a current source interposed between the output of the power source and the electrical circuit. The current source provides a variable resistance conductive path for current flow from the output of the power source to the wiring of a fieldbus circuit. The current source can provide a fixed current output or can be constructed to provide current output-that is adaptable to relatively slow changes in the current requirements of the fieldbus circuit while isolating the output of the power supply from the relatively high frequency digital signals on the bus.
The present invention also provides an apparatus for isolating the input of a direct current voltage converter from voltage variation in the electrical circuit supplying power to the input of the voltage converter comprising a current source interposed between the input of the voltage converter and the electrical circuit.
The adaptive current source has the circuit appearance of a large impedance and permits DC current to pass through the current source while isolating the low impedance constant voltage power supply or, as the case may be, the DC/DC voltage converter from the relatively high frequency digital communication signals on the bus. The current source is more compact and efficient than inductor-based power supply isolators and avoids signal distortion due to ringing in the circuit.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.